This invention relates generally to turbomachines, i.e., turbines, compressors, and similarly bladed fluid flow machines, and more specifically to turbomachine blading.
It is known that the blades of turbomachines are exposed to unsteady flows which produce complex pressure fluctuations or waves of variable intensity. One form of unsteady flow in turbomachines has been described by this inventor in A.S.M.E. Paper No. 66-GT-99, "On a Wave Phenomenon in Turbines," presented at the A.S.M.E. Gas Turbine Conference and Products Show, Zurich, Switzerland, Mar. 15, 1966. This paper discloses a periodic wave phenomenon which occurs in running turbine stages, i.e., two adjacent blade rows, one of which is a stationary row and one of which is a moving row. This wave phenomenon consists of concentrated pressure waves or pulses, which, when conditions are favorable, may be generated in the vicinity of the leading edges of the moving rotor blades, propagate toward the suction side of the stationary stator blades, are reflected back toward the rotor blades, collide with the moving rotor blades in the vicinity of their leading edges, and are again reflected toward the stationary stator blades. This type of unsteady flow occurs most frequently in turbomachines of conventional construction wherein the leading edges of all the blades in a row lie in the same surface of revolution and are equally spaced.
In a turbomachine stage, there may exist a number of different types of reflecting waves. Some waves may travel circumferentially with respect to the stator in the direction of rotation of the turbomachine shaft or in the opposite direction. The reflecting waves, when their number and intensity is large enough, may result in a loss in the operating efficiency of a turbomachine by producing transient or sustained separation of flow from blades in the vicinity of their trailing edges. At the natural frequency of the blades, these reflecting waves may contribute to blade failure. In addition, these waves may contribute to the generation of high intensity noise which is a particularly aggravating. problem in aircraft jet engines.
In order to decrease blade vibration problems and excessive noise generation in turbomachines, selective ratios of the number of stator blades to the number of rotor blades have been studied. While having some merit, such selective ratios have not been able to reduce to acceptable level the reflecting pressure waves. Even if the reflecting waves do not produce any resonant blade excitation at certain operating conditions, they may nevertheless cause flow separation from the blades and generate noise. Similarly, uneven distribution of stator blades along the circumference has been studied. Such uneven stator blade distribution, however, may be dangerous due to introduction of new blade excitation stimuli into the engine. Hence, it would seem that the only solution which ensures an efficient, low noise operation of a turbomachine which is substantially free of resonant blade excitation caused by reflecting waves is one which disrupts these waves without creating further blade excitation stimuli.
One turbomachine blade construction whose object is to disrupt such reflecting waves has been described by this inventor in U.S. Pat. No. 3,347,520. This construction utilizes blades, the leading edges of which are partially or entirely recessed relative to leading edges of other blades lying in the same row. By recessing the leading edges of some stator blades, the pressure waves propagating between the rotor blades and the downstream stator blades are disrupted. Similarly, by recessing the leading edges of some rotor blades, the waves propagating between the stator blades and the downstream rotor blades are disrupted.
As disclosed in U.S. Pat. No. 3,347,520, recessing the leading edges of stator blades may be accomplished with relative ease, by simply machining away a desired portion of selected blades within a row of stator blades. On the other hand, recessing the leading edges of rotor blades, which may have cooling flow passages in them or may be very thin, is much more difficult, since it may necessitate construction of two types of blades for one row: blades without the recessed leading edges and blades with the recessed leading edges.
Compressor designers have also studied other techniques of boundary layer control in axial flow turbomachinery by attempting to optimize blade shape to prevent transition to turbulent flow. One such investigation is disclosed in AGARD Conference Proceedings No. 214 in an article entitled "Secondary Flows in Axial Flow Compressors with Treated Blades" by Boyce. That publication discloses a slotted compressor blade, having a few narrow slots extending across most of the suctionside of the blade, which are said to be effective in reducing boundary layer separation. Applicant believes, however, this configuration would not be effective to disrupt reflecting pressure waves or to attenuate the noise caused by such waves, because the blade surface does not provide sufficient flow disruption. In such a design, the waves can continue to reflect from the unmodified blade surface located between neighboring slots.
It is, therefore, an object of this invention to provide an improved turbomachine component design in which the pressure waves which propagate between the stator blades and the downsteam rotor blades are disrupted without the necessity of having to design two types of blades for at least some rotor rows.
Another object of this invention is to provide an improved turbomachine stator blading construction which will improve blade reliability and machine operating efficiency.
Still another object of this invention is to provide an improved turbomachine which in operation will have a lower intensity of noise than similar machines now in use.